From what I can tell first hand, the tighter more responsive feel may be due to the better condition of the new converter honestly more than what the stall speed is. you can really feel the difference when you gun the throttle, if its is light cruising it feels normal-stockish. From the things I have read though B&M make very good, quality parts and I'm sure the quality has to do much with the performance.
Also taking into account I tuned my carb around the same time, its likely a combination of both that gives an improved feel. but they do go hand in hand.